The idea of building a cross-sea vacuum super high-speed railway has been proposed for a long time. In order to achieve this goal, a safe and reliable ocean tunnel shall be constructed at first.
At present, all the constructed ocean tunnels are submarine tunnels. There are two construction methods. The first method is to excavate a hole in a rock layer or a soil layer at a certain depth from the seabed to build a submarine tunnel (excavation method). This method for constructing the submarine tunnel is long in construction period, difficult in construction and high in project cost, such as Channel Tunnel. The other method is to place the segmented prefabricated tubular structure into a surface layer of the seabed or a shallow soil layer to make water stopping connection and form a submarine tunnel (immersed tube method). This tunnel has been gradually adopted due to its characteristics of construction quality assurance, strong adaptability to geological and hydrological conditions, short construction period, low project cost and the like, such as submarine immersed tube tunnel of Hong Kong-Zhuhai-Macao Bridge. However, the submarine tunnel formed by the immersed tube method and the submarine tunnel formed by the excavation method need to be attached to the submarine soil layer. The depth of the tunnel is affected by the depth of the seabed. If the seabed is deeper, the tunnel is also deeper. Due to the limitation of slope, approaches on two shores of the tunnel will be lengthened, so the project cost will be increased. In addition, the tunnel formed by the immersed tube method needs to be subjected to water stopping connection in water. At a certain depth, water pressure is too high, causing that the technical difficulty of leakage prevention is increased and even the project cannot be implemented due to the limitation of diving depth. Therefore, from the perspectives of economic benefits and technical difficulty, the immersed tube method and the excavation method are not applicable to the construction of tunnels in deep water.
Because of the natural force, i.e., the buoyancy, a submerged floating tunnel has advantages and characteristics that the existing bridges and tunnels do not have: 1. the tunnel adopts an environmental-friendly solution which has small effect on the landform of two shores; 2. the tunnel has the advantage of construction cost compared with the bridges and the tunnels, and the construction cost of the tunnel per unit length is not increased with the increase of span; and 3. the tunnel is not limited by span and water depth, and can be built in steep places with long span and deep water level. Therefore, it is of great significance to study the submerged floating tunnel, which is also the foundation engineering of a cross-sea high-speed railway.
The submerged floating tunnel is floated in water by means of the buoyancy and is supported in three ways: I. a buoy structure: when the buoyancy of a suspended pipeline is small, the buoy is used to increase the buoyancy; II. a riveting cable structure: when the buoyancy of the pipeline is large enough, a riveting cable is used to fix the pipeline; and III. a rigid pile structure: like a bridge in water.
For a high-speed railway tunnel (single track) with an internal diameter of about 5.2 m, the buoyancy of the tunnel is generally greater than the gravity of the tunnel. Therefore, for the cross-sea high-speed railway tunnel, the above supporting modes hardly exist. The supporting modes II and III adopt the riveting cable or a pile column for fixation to the seabed. For the deep sea, construction is almost impossible, just like the immersed tube method mentioned above. In addition, there are also safety problems of anchorage mode and anchorage structure, i.e., vortex-induced vibration (VIV) occurs in an anchorage system under the action of water flow. This has a great influence on the fixation strength of the riveting cable on the seabed.
Besides the above supporting problem, the design and construction of the submerged floating tunnel also have all the problems encountered in ocean engineering, including the following obvious problems: stability of the tunnel under the action of ocean currents, durability of tunnel (material), and safety of structures and personnel. In addition, the tunnel serves the high-speed railway, and the problem of precise control of track flatness necessary for the operation of the super high-speed railway is most important.